The present invention relates to an improved technique for the hydrodealkylation of a hydrodealkylatable hydrocarbon, particularly under conditions of low sulfur, which minimizes carburization thus preventing premature plant shut-downs.
The hydrodealkylation of hydrodealkylatable hydrocarbons such as alkyl aromatics has been practiced for many years. The principal processes involve the conversion of toluene and like alkyl-substituted benzenes to benzene and various byproducts. Such processes are either catalytic or non-catalytic in nature. The catalytic processes employ one or more catalysts that promote the conversion of the alkyl aromatic compounds to benzene and the remaining alkyl. The non-catalytic processes typically employ heat and pressure to promote the conversion of the alkyl aromatic compounds to benzene and the remaining alkyl.
Some conventional catalytic hydrodealkylation processes employ Group VIII metals such as Rh and Pt supported on an alumina support. For example, Kovach et al., in U.S. Pat. No. 3,700,745, describes a hydrodealkylation process which includes contacting an alkyl aromatic hydrocarbon with a catalyst including an active Group VIII metal, such as, platinum, rhodium, palladium, ruthenium and nickel. Other catalytic hydrodealkylation processes employ chromia type catalysts deposited on an alumina support. For example, Daly et al., in U.S. Pat. No. 4,451,687, discloses a catalyst for the hydrodealkylation of alkylaromatic compounds containing chromia on an alumina support. Other catalytic hydrodealkylation processes employ variations of the above catalysts or even completely different catalysts. See, for example, U.S. Pat. Nos. 3,686,340, 3,966,833, 4,189,613, 4,191,632, 4,463,206 and 5,053,574.
The catalytic processes, however, are not always suitable for the commercial conversion of alkyl aromatic compounds to benzene and the remaining alkyl. In particular, the activity, selectivity and conversion rate of such catalysts are not always suitable for large scale hydrodealkylation at the temperatures and pressures suitably employed. If the reaction temperature or pressure is increased, side reactions such as hydrocracking of the aromatic ring is promoted.
Furthermore, some catalysts tend to deactivate with use, presumably due to coke formation on the catalyst surface. In this regard, it is believed that active sites promote polymerization of either hydrogenolysis products or aromatic hydrocarbons resulting in hydrocarbon condensation on the catalyst surface. Under the conditions of the process, these condensed species are dehydrogenated forming coke. The result of these reactions is a reduction in activity of the catalyst since the coke is strongly adsorbed onto the sites which promote dealkylation. In other words, this coke or carbon build-up either blocks or poisons the active catalyst sites causing deactivation. See U.S. Pat. No. 4,451,687.
Additionally, some catalysts tend to deactivate with use, due to the presence of sulfur and in particular thiophene sulfur in the process feed. Thus, catalysts such as certain noble metal catalysts deactivate over time due to the presence of sulfur in the feed. These catalysts must be replaced or regenerated when sulfur reduces the activation to an extent low enough to prevent suitable conversion of the feed.
In view of the disadvantages associated with utilizing catalytic hydrodealkylation processes, non-catalytic hydrodealkylation processes have been developed. Mainly, such processes employ the use of heat and pressure to convert alkylaromatic compounds to benzene and the disassociated alkyl compounds.
Button et al., in U.S. Pat. No. 3,607,960, and Loboda, in U.S. Pat. No. 4,058,452, disclose processes for the thermal hydrodealkylation of an alkyl aromatic, such as toluene, to produce benzene. Both processes include subjecting a gaseous mixture of at least one alkyl aromatic compound and hydrogen in a reaction zone to a reaction temperature in the range of about 1000.degree. to 1800.degree. F. and removing benzene from the effluent. Other patents which disclose the thermal dealkylation of hydrodealkylatable hydrocarbons include U.S. Pat. Nos. 2,929,775, 3,160,671, and 3,284,526.
Thermal hydrodealkylation processes ameliorate the disadvantages associated with the above-mentioned catalytic hydrodealkylation processes in that they do not employ the use of catalysts which are susceptible to deactivation. However, due to the use of high temperatures and pressures which are required for the conversion of alkyl aromatic compounds in the absence of a suitable catalyst, such processes have their own inherent problems.